The two best boxers of our generation are Manny Pacquiao and Floyd Mayweather Jr. Their dominance has been such, with 14 major world titles between them, that there isn't even really a debate. It's simply so.

Both men have walked the same path, parallel journeys to fame and fortune. Across weight classes and against a sprinkling of the same big-name opponents, both have dominated some of the best in the world. More than that, they have done so with the kind of flair that makes mere fighters into something more.

Yet despite occupying twin thrones on the top of the mountain, the two icons have never met in the ring to finally finish their decade-long game of "king of the hill."

There is no easy answer to the question "why?" It's a complicated mix of the personal and the monetary. But the why isn't the most important question facing either man. It's when.

Pacquiao is less than a month away from his 35th birthday. Mayweather will be 37 in February. To trade in the cliche, it's now or never for this fight, or at least a version of this fight that might actually answer the question that has been tickling the brains of boxing fans since both men dispatched the legendary Oscar De La Hoya:

Who's really the man?

There can be only one. That's the whole point of boxing as an exercise, the reason the sport was created. There is little ambiguity in boxing, except who is paying off the judges and when. It's the ultimate binary sport. You win or you lose. Nothing else, when history comes to call you to account, matters.

For years, fans of both men, and their various handlers and mouthpieces, have debated who's to blame for this fight failing to take place. Both sides have compelling cases, but the truth is this—Manny Pacquiao is on the losing side of history.

The top fighters in the world, the men he's most likely to compete with to secure his place, both historically and concurrently, are no longer available to his promoter Bob Arum. Golden Boy, for the time being at least, is winning the boxing war. That means Pacquiao's legacy, long-term, is dependent not just on his own ability to win fights, but on a soon-to-be 82-year-old Arum's ability to manufacture big fights for him out of thin air.

After Pacquiao easily beat Brandon Rios in Macau, it didn't take long for Mayweather's name to pop up in conversation. It's a fight Pacquiao says he's willing to take. Trainer Freddie Roach tells Yahoo's Kevin Iole that it's a fight he thinks Manny can win:

Mayweather is I think slipping a little bit, with his age and so forth. He can't move like he used to. He doesn't use his legs like he used to. He doesn't have the movement. He's not as fluid. He stays on the ropes a lot more. He exchanges a lot more. I think that would benefit Manny Pacquiao in a fight.

Al Bello/Getty Images

Mayweather in action

Team Pacquiao is quick to bring up Mayweather, not just because it's a fight they want, but because opponents are sparse, an issue "Money" can't quite relate to. He has options galore with Golden Boy and Showtime. It's not a fight Golden Boy needs—they have the luxury of picking and choosing from amongst the sport's best, all of them in-house and easy to work with in a way Arum and Pacquiao will never be.

Even Pacquiao's promoter Top Rank and HBO, though he will eventually be left fighting the likes of former sparring partner Ruslan Provodnikov or an overmatched Mike Alvarado, don't need this fight. Opponents can be manufactured and tickets sold with or without Mayweather. They put a fight with Rios, essentially a tune-up bout, on pay-per-view for $59.99. If they can sell that fight, they can sell anything.

Will we ever see Floyd vs. Manny?

No. Boxing politics won't allow it.Yes. The fighters will demand it.Are we still talking about this? It's 2013.Submit Votevote to see results

Will we ever see Floyd vs. Manny?

No. Boxing politics won't allow it.

35.8%

Yes. The fighters will demand it.

35.0%

Are we still talking about this? It's 2013.

29.3%

Total votes: 5,301

Boxing—poor, much-maligned boxing—doesn't need Mayweather-Pacquiao either. The sport hasn't been this healthy in years, with great fights popping up on HBO, Showtime and even network television on a routine basis. The sweet science, a bittersweet business of pain, will march ever onward with or without this bout.

But it is a fight, if both men have their eyes on their legacies, that they need in the worst way. If Pacquiao and Mayweather want to stand among the greats, shoulder to shoulder with Joe Louis and "Sugar" Ray Robinson and the top boxers of this or any era, they will meet in the middle. They simply have to.

Boxing doesn't need Floyd and Manny to fight. They need it. A failure to fight the best, whatever the reason, will hurt them both long-term, even more so than taking on the challenge and failing. Otherwise, for all their various accomplishments—and they are legion—the fight that will define them may be the one that never happened.